Delayed reflow alloy mix solder paste

ABSTRACT

A solder paste mixture for soldering surface mount devices to a circuit board using a reflow soldering process which utilizes a vapor phase furnace. The solder paste mixture has a metallic content which is 63% tin and 37% lead. The metallic content of the paste consists of 150 micron particles of 100% tin and 150 micron particles of an alloy of 10% tin and 90% lead. Included in the process of soldering components to the circuit board is the step of prebaking the circuit board with solder paste and components in their proper place on the board.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a solder paste for soldering surface mountintegrated circuit devices and surface mount discrete devices to circuitboards.

2. Related Art

Surface mount technology is emerging as an important new technique whichrapidly is replacing dual-in-line pin (DIP) packaging for VLSI devices.Surface mount integrated circuit devices can increase considerably thedevice density on a circuit board and they are effective in reducing thetrace and lead lengths, thereby minimizing electrical noise problems.

Surface mount integrated circuit devices come in a variety of differentpackage styles, including J leaded devices such as plastic leaded chipcarriers (PLCCs) and leadless chip carriers (LCCs). These devices oftenare VLSI with high lead counts.

Traditional board soldering methods, such as wave soldering applicableto DIPs, are more expensive when used with surface mount devices, hence,new board soldering equipment has been developed for surface mountdevices. Typically, for large size boards, a solder paste reflow methodis used with a vapor phase furnace. The solder paste usually is amixture of a tin/lead solder alloy with rosin and well-known solvents.The composition of the solder alloy typically is 63% tin and 37% lead,which is the tin/lead alloy composition with the lowest possible meltingtemperature. That is, 63% tin and 37% lead is the eutectic alloy of tinand lead. This alloy has a melting temperature of 183 degrees C. Thesolder paste is applied to all points (footpads) on a circuit board towhich the leads of surface mount devices are to be soldered. The surfacemount devices are then placed upon the circuit board which thereafter ispositioned in a vapor phase furnace. In the furnace, the board, thesurface mount device leads and the solder paste are rapidly heated tomelt the solder to fuse the device leads to the footpads. The furnacetemperature is 215 degrees C.

The present day solder paste reflow method is susceptible to producing ahigh proportion of open solder joints for J leaded integrated circuitdevices, and weakened solder joints containing voids and cavities formost surface mount package styles.

An open solder joint between a device lead and a circuit board footpadoccurs when most or all of the solder cn the footpad wicks up along thelength of the lead, leaving an insufficient quantity of solder at thepad to form a solder joint or fillet between the pad and lead. Solderwicking takes place when solder wetting forces on the lead are strongerthan those on the footpad, causing the solder to be drawn to the leadsurfaces more strongly than to the pad surface. This can happen whensolder wetting occurs before or more strongly on the lead surfaces thanit does on the footpad surface, and where the lead is sufficiently farfrom the surface of the pad.

High lead count, VLSI, surface mount devices are susceptible to leadaplanarity. Leads may be out of plane with each other by as much asseveral mils. Experience has shown that when a lead is farther thanabout one mil from the surface of a footpad, wicking sufficient to causean open solder joint can take place. Also, it has been found thatapproximately five percent of the open solder joints which do occur,happen when lead/footpad separations are between one and two mils, andapproximately ninety-five percent of the opens occur when separationsare greater than two mils.

In addition to lead/footpad separations of from one to several mils,stronger solder wetting of the lead than the footpad is required for anopen solder joint to occur. FIG. 1 shows the temperatures of a typicalcircuit board and of the leads of a J lead device after insertion into avapor phase furnace. As seen in FIG. 1, the temperature of the deviceleads rises from 183 degrees C. (melting point of eutectic solder, andthe temperature at which wetting begins) to the furnace temperature of215 degrees C. more quickly than does the temperature of the circuitboard (including the footpads mounted on the board). This occurs becausethe thermal mass of the circuit board is much greater than that of theleads. Since solder wetting occurs more quickly on hotter surfaces thanon cooler ones, it is evident from FIG. 1 that solder wetting will occurmore quickly on the lead surfaces than on the footpad surface, thusgiving rise to stronger wetting forces attracting the solder to the leadsurfaces than to the footpad surface. When the lead/footpad separationis sufficiently great, the wetting forces are dominant and sufficientwicking takes place to form an open solder joint.

Experience with the present day method of reflow soldering J leaddevices in a vapor phase furnace has shown that the frequency ofoccurrence of open solder joints varies from approximately 250 to 2,000per million solder joints attempted. 1,000 bad joints per millionattempted gives a ratio of one bad joint to every 1,000 attempted. Manylarge modern circuit boards have several thousand joints per board,thus, it is possible to have a zero yield of perfect boards, leading toconsiderable costs in trouble shooting and rework.

The unwanted creation of voids and cavities in the solder joints betweenthe contacts of surface mount integrated circuit devices and thefootpads on the circuit board is another drawback of the present daymethod of solder reflow in a vapor phase furnace. Most conventionalsolder pastes outgas solvents and flux vehicles during reflow, ormelting of the solder. Unfortunately, some of these gases continue to beproduced after the surface layer of solder has melted, and since liquidsolder has a very high surface tension which prevents the gases fromescaping, the gases are trapped within the joint as voids. Investigationhas shown that the voids weaken the solder joint, making it from ten toone thousand times more susceptible to fatigue and cracking, especiallywhere leadless ceramic chip carriers are involved.

SUMMARY OF THE INVENTION

The invention provides a solder paste mixture containing two kinds ofparticles which, when taken together, give the metallic content of themixture a net composition of 63% tin and 37% lead, by weight. One of thekinds of particles consists of particles of 10% tin and 90% lead. Theother kind of particle is 100% tin. Thus, as a percentage by weight ofthe total metal content in the mixture, there is approximately 41% ofthe 10% tin/90% lead kind of particles, and 59% of the 100% tin kind ofparticles. Also provided by the invention is the process step ofprebaking the circuit board with solder paste on its footpads, prior tothe soldering step in the vapor phase furnace.

Whereas present day eutectic solder pastes begin melting right away at183 degrees C., the solder paste of this invention does not beginsubstantial melting until approximately twelve seconds after it hasreached 183 degrees C. The time to melt is proportional to the size ofthe particles, which enables the time delay before melting to be varied.As seen in FIG. 1, the board takes approximately ten seconds more toreach the eutectic melting point of 183 degrees C. than do the leads,thus, a delay of twelve seconds ensures that the wetting forces on thefootpads will be sufficient to prevent wicking of the solder up theleads, thereby preventing or lessening considerably the number of opensolder joints produced during reflow soldering in a vapor phase furnace.Testing of the invention has shown that where the leads are no fartherthan three mils from their footpads, open solder joints are virtuallyeliminated. In addition, it has been shown that the invention hasprevented open solder joints for lead/footpad separations of up to eightmils.

The invention also is effective for increasing the strength of thesolder joints and making them less susceptible to fatigue. This isaccomplished by driving off most of the volatile substances in thesolder paste prior to solder melting at the surface of the joint,thereby producing fewer and smaller voids in the joint. The time delayprovided by the solder paste mixture of the invention allows gases toescape from the solder mixture, during the period of the delay, afterthe eutectic melting point of 183 degrees C. has been reached.Furthermore, the invention provides for a prebake step which takes placeprior to insertion of the circuit board into the vapor phase furnace.The prebake step calls for heating the circuit board with the solderpaste already on the footpads to allow volatile substances to outgasfrom the solder paste mixture. Prior to the prebake step, the surfacemount integrated circuit devices which are to be soldered to the board,are placed on the board in the positions in which they will be soldered.At this point, the leads or contacts of the devices are in contact withthe solder paste on the footpads so that, as the paste hardens duringprebake, the hardened paste mechanically holds the devices in the properpositions for the soldering process in the vapor phase furnace.

The amount of void reduction which takes place has been quantitativelyevaluated by comparing voids in joints produced using a conventionaleutectic solder paste, with voids produced using the solder paste ofthis invention. After assembly, test devices were lifted off the circuitboard, exposing the voids within the solder joints. The diameter of thelargest void in each solder joint was then measured using an opticalscope with an electronic micrometer. Eighty joints produced using theconventional solder paste were compared with eighty joints producedusing the solder paste of this invention, with the result being that themean maximum void diameter was 1.5 mils for the solder paste of thisinvention and 4.6 mils for the conventional solder paste. Consideredvolumetrically, the solder paste of this invention reduced the meanmaximum void volume by approximately 97%.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing the temperature of a typical surface mountcircuit board and the temperature of the leads on a J lead integratedcircuit surface mount device, both plotted versus the time afterinsertion of the board and surface mount device into a vapor phasefurnace.

FIG. 2 is magnified view of a section of the solder paste mixture ofthis invention.

FIG. 3 shows the tin/lead alloy phase diagram.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Turning to FIG. 2, the solder paste mixture 1 is shown in whichspherical particles of tin 2 and 10% tin/90% lead 3 are mixed togetherwith and suspended in a tacky medium 4. The tin particles 2 are 100% tinwhile the 10% tin/90% lead particles 3 are an alloy of 10% tin and 90%lead, by weight. The tacky medium 4 is conventional for solder paste andcontains solder rosin and well known solvents. The ratio of tinparticles 2 to 10% tin/90% lead particles 3 is such, that for themetallic (solder) portion of the mixture 1, the net percentage of tin is63% while the net percentage of lead is 37%. That is, taken as a whole,the metallic portion of the mixture 1 contains the proper percentages oftin and lead to form an eutectic alloy of tin and lead if all the tinand lead in the mixture were alloyed together. Thus, by weight,approximately 41% of the the metallic portion of the mixture is the 10%tin/90% lead alloy and 59% is the pure tin.

As seen in FIG. 3, eutectic solder melts at 183 degrees C. at 5, and isseen to be an alloy of 63% tin and 37% lead. This temperature is belowthe vapor phase furnace temperature of 215 degrees C. indicated at 6.Pure tin melts at 231 degrees C., shown at 7, while a 10% tin/90% leadalloy begins to melt at 263 degrees C., shown at 8, thus, it is seenthat both melting points are above the vapor phase furnace temperatureof 215 degrees C. indicated at 6. Therefore, if the metallic portion ofa solder paste consists of pure tin particles 2 alone or 10% tin/90%lead alloy particles 3 alone, the paste would not melt in a vapor phasefurnace. However, the solder paste mixture 1 of this invention will meltat the vapor phase furnace temperature, indicated at 6, even though,taken separately, neither of the two constituent types of particleswould melt. This is possible because solid state diffusion at theinterfaces between tin particles 2 and 10% tin/90% lead particles 3 willform an alloy at these interfaces which is fusible at the eutectic alloytemperature. In this manner, although all the particles of metal willeventually melt, the melting will be delayed and will not occur asquickly as it would if all the particles were of the eutectic alloy oftin and lead. As seen in FIG. 1, the time delay for melting is measuredfrom the time 9 at which the circuit board reaches the eutectic alloymelting temperature of 183 degrees C.

The amount of time by which melting is delayed is controllable by therate of solid state diffusion which in turn is controllable by the sizeof the particles making up the metallic portion of the solder pastemixture 1. The melting rate increases or decreases when the diffusionrate increases or decreases, and the diffusion rate increases ordecreases when the particle size increases or decreases. Therefore, theamount of time delay afforded is engineerable through control of theparticle size. The preferred size of the particles is 100 microns whichaffords a time delay of approximately twelve seconds if a prebaking stepis used before reflow (solder melting) in a vapor phase furnace.

The solder paste mixture 1 is applied to a circuit board in the samemanner as is done with conventional solder pastes. That is, the solderpaste mixture 1 is applied to a circuit board by screen printing, whichensures that the solder paste mixture 1 is applied only to the footpadsof the board. The thickness of the paste upon the footpads is ten totwelve mils. After the solder paste mixture 1 has been applied to thefootpads, surface mount devices, such as integrated circuits anddiscrete components, are placed upon the board, preferrably bypick-and-place machines, and then the board is prebaked at a temperatureof 105 degrees C. for a period lasting from one to four hours.

The prebake step accomplishes at least two things: it lengthens the timedelay obtainable for a given size of particle 2 and 3, and it reducesthe number and size of voids created in the solder joints.

Prebake eliminates many of the volatile substances in the solder pastemixture 1 which otherwise would gassify during the soldering process inthe vapor phase furnace. Gasses released during heating in the vaporphase furnace agitate the metal particles 2, 3 and increase the rate ofsolid state diffusion, thereby causing the metal particles 2, 3 to meltat a faster rate. Thus, the time delay in melting can be lengthened bydecreasing the amount of volatile material in the solder paste mixture 1prior to heating in the vapor phase furnace.

I claim:
 1. A solder paste mixture for delaying solder melt andminimizing solder joint voids, comprising:a plurality of particles of afirst alloy, and a plurality of particles of a second alloy differingfrom the first alloy, wherein the first alloy includes tin and thesecond alloy includes lead.
 2. The mixture of claim 1 wherein the secondalloy also includes tin.
 3. The mixture of claim 2 wherein the amount oftin in the mixture is approximately sixty-three percent by weight andthe amount of lead in the mixture is approximately thirty-seven percentby weight, of the particle in the mixture.
 4. The mixture of claim 3wherein the first alloy is one hundred percent tin and the second alloy,by weight, is approximately ten percent tin and ninety percent lead. 5.The mixture of claim 4 further comprising:a tacky medium in which theparticles of the first and second alloys are suspended.
 6. A solderpaste for delayed melting in a vapor phase furnace, in which themetallic content of the paste comprises:tin particles, and alloyparticles of an alloy of approximately ten percent tin and ninetypercent lead by weight.
 7. The solder paste of claim 6 wherein the ratioof tin particles to the alloy particles is such that, by weight, themetallic content of the paste is approximately sixty-three percent tinand thirty-seven percent lead.
 8. The solder paste of claim 7 in whichthe tin particles and alloy particles are approximately one hundredmicrons in diameter.
 9. A solder mixture for delaying solder melt,comprising:a plurality of metal first alloy particles which include tin,and a plurality of metal second alloy particles which include lead,wherein the first alloy differs from the second alloy, and the metalliccontent of the mixture by weight is approximately sixty-three percenttin and thirty-seven percent lead.
 10. The solder mixture of claim 9,wherein the first alloy is approximately one hundred percent tin and thesecond alloy is approximately ten percent tin and ninety percent lead.11. The solder mixture of claim 10 wherein all the particles areapproximately spherical in shape.
 12. The solder mixture of claim 10wherein the particles are sized to provide a time delay of approximatelytwelve seconds from the time that the mixture has reached the meltingtemperature of a eutectic alloy of tin and lead.
 13. A solder mixturefor delaying solder melt, comprising:a plurality of metal first alloyparticles which include tin, and a plurality of metal second alloyparticles which include lead, wherein the first alloy differs from thesecond alloy, and the metallic content of the mixture by weight is suchthat the ratio of tin to lead is the same as for a eutectic alloy of tinand lead.
 14. The mixture of claim 13, wherein the first and secondalloy particles are substantially spherical in shape.
 15. The mixture ofclaim 14, wherein the first and second alloy particles are approximatelyone hundred microns in diameter.
 16. The mixture of claim 13, whereinthe first and second alloy particles are suspended in a tacky medium.17. The mixture of claim 13, wherein the first alloy is approximatelyone hundred percent tin and the second alloy is approximately ninetypercent lead and ten percent tin by weight.
 18. The mixture of claim 13,wherein the particles are sized to provide a melting delay time ofapproximately twelve seconds after the mixture has reached the meltingtemperature for a eutectic alloy of tin and lead.